<PAGE>   1
                                                                    EXHIBIT 10.6

                         MASTER SECURED PROMISSORY NOTE
                                (Line of Credit)

$15,000,000                   Greeneville, Tennessee          September 10, 1998

         FOR VALUE RECEIVED, on or before September 10, 2000 (the "Maturity
Date"), the undersigned, LANDAIR CORPORATION, a Tennessee corporation (referred
to herein as "Maker"), promises to pay to the order of FIRST TENNESSEE BANK
NATIONAL ASSOCIATION, a national banking association organized under the laws of
the United States of America ("Payee"; Payee and any subsequent holder[s] hereof
are hereinafter referred to collectively as "Holder"), without grace except as
provided for herein, at the office of Payee at 2841 Andrew Johnson Highway,
Greeneville, Tennessee 37745, or at such other place as Holder may designate to
Maker in writing from time to time, the principal sum of FIFTEEN MILLION AND
NO/100THS DOLLARS ($15,000,000), or such other amount as may hereafter be
outstanding hereunder, whichever is less, together with interest on the
outstanding principal balance hereof from date at LIBOR plus the Applicable
Margin, as calculated and adjusted in accordance with the Loan Agreement (as
hereinafter defined); provided that in no event shall the rate of interest
payable in respect of the indebtedness evidenced hereby exceed the maximum rate
of interest from time to time allowed to be charged by applicable law (the
"Maximum Rate"). Interest shall be calculated on the basis of a 360-day year for
each day that all or any part of the indebtedness evidenced hereby shall be
outstanding, to the extent permitted by applicable law.

         Interest only on the outstanding principal balance hereof shall be due
and payable monthly, in arrears, with the first installment being payable on the
first (1st) business day of the first month after the date hereof, and
subsequent installments being payable on the same day of each succeeding month
thereafter until the Maturity Date, at which time the entire outstanding
principal balance, together with all accrued and unpaid interest, shall be due
and payable in full.

         All payments in respect of the indebtedness evidenced hereby shall be
made in collected funds, and shall be applied to principal, accrued interest and
charges and expenses owing under or in connection with this Note in such order
as Holder elects, except that payments shall be applied to accrued interest
before principal.

         The indebtedness evidenced hereby may be prepaid only in accordance
with the provisions of the Loan Agreement.

         Any advance by Payee to Maker that is not evidenced by another
instrument or agreement between the parties shall be conclusively presumed to
have been made hereunder when such advance is either (1) deposited or credited
to an account of Maker with Payee, notwithstanding that such advance was
requested, orally or in writing, by someone other than Maker or that someone
other than Maker is authorized to draw on such account and may or does withdraw
the whole or part of such advance, or (2) made in accordance with the oral or
written instructions of Maker. The entire balance of all advances hereunder that
may be outstanding from time to time shall constitute a single indebtedness, and
no single advance increasing the outstanding balance hereof 


<PAGE>   2
shall itself be considered a separate loan, but rather an increase in the
aggregate outstanding balance of the indebtedness evidenced hereby.

         Time is of the essence of this Note. It is hereby expressly agreed that
in the event that any Event of Default, as defined in that certain Loan and
Security Agreement of even date herewith, by and between Maker and Payee (the
"Loan Agreement"), shall occur; or should any default or event of default occur
under any other instrument or document now or hereafter evidencing, securing or
otherwise relating to the indebtedness evidenced hereby, subject to applicable
cure periods; then, and in such event, the entire outstanding principal balance
of the indebtedness evidenced hereby, together with any other sums advanced
hereunder, under the Loan Agreement or under any other instrument, document or
agreement now or hereafter evidencing, securing or in any way relating to the
indebtedness evidenced hereby, together with all unpaid interest accrued
thereon, shall, at the option of Holder and without notice to Maker, at once
become due and payable and may be collected forthwith, regardless of the
stipulated date of maturity. Upon the occurrence of any default as set forth
herein, at the option of Holder and without notice to Maker, all accrued and
unpaid interest, if any, shall be added to the outstanding principal balance
hereof, and the entire outstanding principal balance, as so adjusted, shall bear
interest thereafter until paid at a rate (the "Default Rate") equal to the
lesser of (i) the rate that is four percentage points (4%) in excess of Payee's
Base Rate, as it varies from time to time, or (ii) the Maximum Rate, regardless
of whether there has been an acceleration of the payment of principal as set
forth herein. All such interest shall be paid at the time of and as a condition
precedent to the curing of any such default.

         To the extent permitted by applicable law, Maker shall pay to Holder a
late charge equal to four percent (4%) of any monthly payment hereunder that is
not received by Holder within fifteen (15) days of the date on which it is due,
in order to cover the additional expenses incident to the handling and
processing of delinquent payments; provided, however, that nothing in this
provision shall be deemed to waive any other right or remedy of the Holder
hereof by reason of Maker's failure to make payments when due hereunder.

         In the event this Note is placed in the hands of an attorney for
collection or for enforcement or protection of the security, or if Holder incurs
any costs incident to the collection of the indebtedness evidenced hereby or the
enforcement or protection of the security, Maker and any indorsers hereof agree
to pay a reasonable attorney's fee, all court and other costs, and the
reasonable costs of any other collection efforts.

         Presentment for payment, demand, protest and notice of demand, protest
and nonpayment are hereby waived by Maker and all other parties hereto. No
failure to accelerate the indebtedness evidenced hereby by reason of default
hereunder, acceptance of a past-due installment or other indulgences granted
from time to time, shall be construed as a novation of this Note or as a waiver
of such right of acceleration or of the right of Holder thereafter to insist
upon strict compliance with the terms of this Note or to prevent the exercise of
such right of acceleration or any other right granted hereunder or by applicable
laws. Unless otherwise specifically agreed by Holder in




                                        2

<PAGE>   3
writing, the liability of Maker and all other persons now or hereafter liable
for payment of the indebtedness evidenced hereby, or any portion thereof, shall
not be affected by (1) any renewal hereof or other extension of the time for
payment of the indebtedness evidenced hereby or any amount due in respect
thereof, (2) the release of all or any part of any collateral now or hereafter
securing the payment of the indebtedness evidenced hereby or any portion
thereof, or (3) the release of or resort to any person now or hereafter liable
for payment of the indebtedness evidenced hereby or any portion thereof. This
Note may not be changed orally, but only by an agreement in writing signed by
the party against whom enforcement of any waiver, change, modification or
discharge is sought.

         The indebtedness and other obligations evidenced by this Note are
further evidenced and/or secured by (1) the Loan Agreement, (2) the Guaranty
Agreement dated as of the dated hereof executed by Landair Transport, Inc.,
Volunteer Adjustment, Inc. and Landair Transportation Properties, Inc. (the
"Guarantors"), (3) the Security Agreement dated as of the date hereof executed
by the Guarantors, (4) a Pledge and Security Agreement dated as of the date
hereof executed by the Maker, and (5) certain other instruments and documents as
more particularly described in the Loan Agreement.

         All agreements herein made are expressly limited so that in no event
whatsoever, whether by reason of advancement of proceeds hereof, acceleration of
maturity of the unpaid balance hereof or otherwise, shall the interest and loan
charges agreed to be paid to Holder for the use of the money advanced or to be
advanced hereunder exceed the maximum amounts collectible under applicable laws
in effect from time to time. If for any reason whatsoever the interest or loan
charges paid or contracted to be paid in respect of the indebtedness evidenced
hereby shall exceed the maximum amounts collectible under applicable laws in
effect from time to time, then, ipso facto, the obligation to pay such interest
and/or loan charges shall be reduced to the maximum amounts collectible under
applicable laws in effect from time to time, and any amounts collected by Holder
that exceed such maximum amounts shall be applied to the reduction of the
principal balance remaining unpaid hereunder and/or refunded to Maker so that at
no time shall the interest or loan charges paid or payable in respect of the
indebtedness evidenced hereby exceed the maximum amounts permitted from time to
time by applicable law. This provision shall control every other provision in
any and all other agreements and instruments now existing or hereafter arising
between Maker and Holder with respect to the indebtedness evidenced hereby.

         This Note has been negotiated, executed and delivered in the State of
Tennessee, and is intended as a contract under and shall be construed and
enforceable in accordance with the laws of said state, except to the extent that
Federal law may be applicable to the determination of the Maximum Rate.

         As used herein, the terms "Maker" and "Holder" shall be deemed to
include their respective successors, legal representatives and assigns, whether
by voluntary action of the parties or by operation of law. In the event that
more than one person, firm or entity is a maker hereunder, then all references
to "Maker" shall be deemed to refer equally to each of said persons,




                                        3

<PAGE>   4


firms, or entities, all of whom shall be jointly and severally liable for all of
the obligations of Maker hereunder.

         IN WITNESS WHEREOF, the undersigned Maker has caused this Note to be
executed by its duly authorized officer as of the date first above written.


                                        MAKER:


                                        LANDAIR CORPORATION


                                        By:    /s/   E. R. Brown
                                               ---------------------------------
                                        Title: President
                                               ---------------------------------




                                        4